Lung cancer is the leading cause of cancer deaths, and is largely refractory to standard chemotherapy. Selective EGFR inhibitors (e.g., erlotinib) elicit clinical responses in 10-20% of non-small cell lung cancers (NSCLC), which correlates with activating EGFR mutations, However, there remains a large fraction of patients for which erlptinib, as monotherapy, is ineffective. EGFR is expressed in most NSCLCs, suggesting that it may still be an important therapeutic target, in conjunction with additional treatment. We propose to expand the clinical utility of erlotinib by identifying a rational combination with a second treatment to benefit an additional subset of NSCLC patients. Aim 1: To establish the efficacy of erlotinib in combination with a second treatment. By utilizing 108 NSCLC erlotinib-refractory cell lines, we will test the ability of erlotinib plus an additional treatment to produce cytostatic or cytotoxic activity. We will focus on inhibitors of the MET and IGF-1, kinases, as well as HSP90. We will also test the abiHty of ionizing radiation to synergize with erlotinib. Together, these studies are expected to reveal subsets of NSCLCs that are sensitive to the proposed combination therapies. Aim 2: To establish mechanisms by which combination treatment with erlotinib and a second agent inhibits cell survival. In NSCLC cell lines with synergistic response to combination treatment, we will test the hypothesis that inhibition of survival pathways that may be redundant to EGFR-derived signals produces synthetic lethality. We will also characterize radiation-induced EGFR activation in NSCLCs that are sensitive to erlotinib and radiation, and we will confirm that such EGFR activation is also seen in NSCLC patient explants. We hypothesize that cell death observed in highly sensitive cell lines resembles the previously described apoptotic response to erlotinib in cells with EGFR mutations. Aim 3: To identify jiomarkers that predict sensitivity to treatment combinations. In cell lines sensitive to combination treatment, we will correlate sensitivity with: (i) recurrent gene mutations in NSCLC, (ii) comparative genome hybridization array data, and (iii) gene expression profiles. Our findings are expected to inform clinical trials for NSCLC patients with acquired erlotinib/gefitinib resistance (in development |n Projects 4 and 5 of this SPORE) and ultimately lead to genotype-driven trials of novel drug combinations or molecularly targeted radiation therapy in patients whose tumors exhibit primary erlotinib resistance.